1. Field of the Invention
The present invention relates to the removal of mineral pollutants from water and, more particularly, to an improved method for recovery of spent regenerants and removal of mineral salts therefrom.
2. Description of the Prior Art
Water is becoming an increasingly scarce natural resource and with increasing population and industrial and commercial use of water, ionic pollution in the form of waste and by-product streams from industrial plants, refineries and municipal sewage is having a drastic effect on the ecological balance of lakes, rivers streams and even the oceans. Ionic pollution is found to destroy the food chain of marine life and in some cases so upsets the biological balance as to cause hazards to swimmers and fishermen. Even low ionic pollution in irrigation waters can have a drastic effect in terms of the accumulation of salts in the soil over extended periods of time. One of the main concerns of the present invention is to provide a method for demineralizing industrial and other waste streams prior to discharge into surface waterways. Water can be demineralized by many processes such as distillation, reverse osmosis, chemical precipitation or ion exchange. These processes, though all practiced commercially, suffer from limitations such as scaling, poor economics, and excessively high total dissolved solids (TDS) in the effluent, or the production of ecologically undesirable waste streams such as concentrated brines which are both expensive and difficult to dispose.
Since the development of commercially practicable synthetic ion-exchange resins, ion-exchange techniques have been the preferred methods for demineralizing water because of the high purity water produced, i.e., low TDS. However, the chemical costs for these systems have been quite high per unit of various salts removed. The ion-exchange method which conventionally has been used consisted of the use of beds of strong acid cation exchanger in the hydrogen form. The resin must be regenerated with a strong acid or strong base, depending upon the nature of the chosen resin. In these prior art systems, regeneration requires a considerable excess of regenerant and the original reaction does not proceed to completion, even though the regeneration may.
Deionization processes employing weak acid and weak base resins are per se known to the art. Two very real advantages in using these weak resins are the achievement of very high regeneration efficiencies, and a high theoretical loading capacity. Both types of weak exchangers can easily and effectively be regenerated to high levels by employing amounts of regenerant only slightly in excess of stoichiometry.
An improved process for the removal of mineral pollutants from water is disclosed and claimed in U.S. Pat. No. 3,700,592 issued on Oct. 24, 1972. This process comprises the following steps:
A. Passage of the aqueous mineral solution through weakly acidic cationic resin and weakly basic anion resin contained in a mixed bed to remove the mineral salts and provide a salt-free product stream.
B. Separation of the spent cationic and anionic resins.
C. Regeneration of the cationic resin with a solution of a chelating agent.
D. Regeneration of the anionic resin with an organic solution of a base.
E. Contacting the cationic regenerant stream (metal chelate solution) with carbon dioxide to precipitate metal carbonates and recover the cationic regenerant for recycle.
F. Contacting the above metal carbonates with the anionic regenerant stream (organic solution of amine salts) at elevated temperatures to precipitate metal salts and recover carbon dioxide and the amine regenerant for recycle.
The above-described process requires that the two regenerant streams are separated during the salt precipitation step. Furthermore, the mineral salts are separately recovered as metal carbonates from the cation regenerant stream and as a metal salt from the anion regenerant stream and the process is unnecessarily complex.